U.S. citizens who are non-residents of Canada but have Canadian source income must file a T1-NR Individual Tax Return with the CRA.1 U.S. residents who are non-residents of Canada and earn only passive income need not file a Canadian tax return. Instead, a withholding tax is withheld by the payor and remitted directly to the CRA on behalf of the non-resident. This requirement imposed on payers of monies going to non-residents simplifies the CRA tax collection efforts as against non-residents.
1. https://www.canada.ca/en/revenue-agency/services/forms-publications/tax-packages-years/general-income-tax-benefit-package/non-residents.html (2022 Income Tax and Benefit Package (for non-residents and deemed residents of Canada)).